This invention is related to telephone line circuits and more particularly to the reduction of power dissipation and the elimination of inductors in such circuits.
A telephone line circuit is interposed between the PBX line and the associated telephone station. These circuits serve, among other things, to provide both AC and DC power to activate the telephone transmitter and receiver. These circuits presently contain inductors which serve to isolate the DC power supply and to allow for the circuit's AC impedance to differ from the DC resistance. One major aim of circuit designers has been the elimination of the costly, and bulky, inductors from the line circuits, thereby making the circuits fully resistive.
Resistive line feed circuits, however, have not been popular in the past for several reasons, namely because they do not reject power supply noise. Any noise on the battery supply shows up as an audible signal in the telephone. A second problem is that ground potential noise can be heard as an audible signal. Another problem with resistive-only line circuits is that one port could crosstalk to others through the power supply internal impedance.